A lighting circuit (ballast) for supplying power in a stable manner is used for turning ON a discharge lamp such as a metal halide lamp to be used in a headlamp for a vehicle. For example, a discharge lamp lighting circuit disclosed in Japanese Patent Document JP-A-2005-63821 comprises a DC/AC converting circuit including a series resonant circuit, and an AC power is supplied from the DC/AC converting circuit to a discharge lamp. The magnitude of a supply power is controlled by changing a driving frequency of the series resonant circuit.
Moreover, the discharge lamp lighting circuit also controls turning ON the discharge lamp. More specifically, the discharge lamp lighting circuit controls a no-load output voltage (OCV: Open Circuit Voltage) before turning ON the discharge lamp and applies a high voltage pulse to the discharge lamp to turn ON the discharge lamp, and then carries out a transition to a stationary lighting state while reducing a transient input power.
FIG. 16 is a graph showing a conceptual relationship between the driving frequency of the series resonant circuit and the magnitude of the supply power (or OCV). In FIG. 16, a graph Ga shows a relationship between the driving frequency and OCV before a lighting operation and a graph Gb shows a relationship between the driving frequency and the supply power after the lighting operation. As shown in FIG. 16, the magnitude of the supply power (or OCV) to the discharge lamp has a maximum value when the driving frequency is equal to a series resonant frequency (before the lighting operation: fa; after the lighting operation: fb), and is decreased when the driving frequency becomes greater (or smaller) than the series resonant frequency. In a region in which the driving frequency is lower than the series resonant frequency, a switching loss is increased so that a power efficiency is reduced. The magnitude of the driving frequency is controlled in a region in which the driving frequency is higher than the series resonant frequency.
In controlling lighting of the discharge lamp, an operating point before the lighting operation is set to a point Pa corresponding to a driving frequency fc higher than the series resonant frequency fa and an operating point after the lighting operation is set in a region X which is higher than the series resonant frequency fb. In a conventional discharge lamp lighting circuit, a transition from the point Pa to the region X is carried out in the following manner, for example. More specifically, after the discharge lamp is turned ON at the operating point Pa, the driving frequency fc before the lighting operation is maintained for a certain time. At this time, a correlation between the driving frequency and the supply power is changed to the graph Gb. Therefore, a transition of the operating point to a point Pc is carried out. Then, the driving frequency is changed by a predetermined variation Δf (=fd−fc) to carry out a transition of the operating point to the point Pb in the region X.
However, it is difficult to set the frequency variation Δf in view of a fluctuation in a source voltage, a variation in an operating temperature and an error of an electrical characteristic of an electronic component. The characteristic of an electronic component to be used in the discharge lamp lighting circuit has a variation and a difference between the resonant frequencies before and after the lighting operation (fb−fa) is varied. In order to preset Δf, accordingly, it is necessary to design a component with a margin or to regulate Δf for every individual part. When the design of the component is designed to have the margin, however, an overspec is generated, which is undesirable. Even if Δf is regulated for every individual part, moreover, there is a possibility that a lighting characteristic might be deteriorated with the initial Δf when a characteristic of a circuit is changed due to aging deterioration.